Buried-channel CCD's have often required a bias or fat-zero charge to successfully transfer very small charge packets. The fat-zero applied, usually generated a shot noise component that was higher than the noise produced by the sensor's on-chip amplifier. The Texas Instruments 800.times.800 3-phase CCD for example, requires a fat-zero of 100 electrons (e-) to fill in a design-induced trap associated with its transfer gate region. Although the on-chip amplifier noise for the Texas Instruments device is only 6e-rms, the fat-zero required for complete transfer, generates a shot noise of 10e-, increasing the overall noise floor of the detector to 11.6e-.
Recent efforts by CCD manufacturers have made notable progress in eliminating design and process-induced trapping centers, similar to those experienced with the noted Texas Instrument CCD. Today, charge transfer efficiency (CTE) performance is usually limited by bulk state traps, small electron traps found naturally in the bulk silicon on which the CCD is made. Current CCD's exhibit near perfect charge transfer efficiency as a result of the high quality silicon that is grown today. For example, a "bulk state limited" CCD can transfer a 10,000 e- charge packet 521 transfers with less than 5 e- deferred without the aid of a fat-zero charge.
Although charge packets of a few electrons can be transferred, the CCD is unable to read the charge accurately because of the relatively high noise floor inherent to the sensor's on-chip amplifier (typically a few electrons rms). Recently, efforts have been directed by CCD manufacturers to break the 1 e- noise barrier so that the high CTE now achieved can be fully exploited.
Theoretically, the read noise for a scientific CCD can be reduced without limit by the amount of process time spent on each pixel. There are, however, practical limits to this procedure. Employing short sample periods, of typically less than 8 microseconds, the noise of CCD's decreases by the square-root of the sample time. However, for longer sample times, the noise only gradually decreases and for some CCD camera systems, the noise actually increases, due to low frequency noise sources encountered (e.g., 1/f noise generated by the CCD amplifier). Current knowledge in minimizing CCD amplifier noise indicates that 2-3 e-noise levels may be the practical limit assuming that conventional output charge detection schemes are utilized (i.e. floating diffusion MOSFET amplifiers).
A book entitled Charge Transfer Devices, by Sequin and Thompsett, published by the Academic Press, Inc. in 1975, discloses at pages 56 and 57 thereof, the concept of utilizing a floating gate amplifier to sense charge transferred in a CCD, non-destructively. They describe what they call a distributed amplifier concept, in which a large number of floating gate amplifiers may be used and their respective signals combined to increase signal-to-noise ratio by a factor equal to the square-root of the number of such amplifier stages. Unfortunately, improving signal to noise ratio in CCD's using such a distributed amplifier scheme is not practical. For one thing, the use of a large number of amplifiers is too complicated, particularly where the signal-to-noise ratio must be improved by factors approaching 100, for example, where 10,000 such amplifiers would have to be provided for processing each pixel. Even if one considers only the additional surface area required to provide such a large number of floating gate amplifiers, the sacrifice for providing a significant improvement in signal-to-noise would be prohibitive. Furthermore, even ignoring the additional surface area required, one then has to confront the problem resulting from offset differences between amplifiers. Providing 10,000 or more floating gate amplifiers on a single integrated circuit chip, all of which amplifiers have identical gains, would be prohibitively expensive and time consuming and thus impractical.
There is therefore an ongoing need for a buried-channel CCD structure which circumvents the 1/f noise problem to provide a square-root reduction in noise and to allow sub-electron noise floors to be realized, but without requiring the use of a large plurality of amplifiers which would otherwise create an impractical circuit configuration.